This invention relates generally to the lost pattern process of investment casting, and more specifically to the formation of the pattern assemblies used in such a process.
In the lost pattern process, a pattern assembly is conventionally prepared by attaching a plurality of patterns around a cylindrical sprue or center tree to form a branched "set-up." A pouring cup or well is frequently attached to the cylindrical center tree. The patterns, which are replicas of the parts to be cast in metal and include the necessary gates and risers, are made of an expendable material, such as wax, synthetic resin or a wax and synthetic resin composition. The pattern set-up is coated or invested with a suitable slurry which is allowed to harden to form a refractory mold. Thereafter, the patterns are destroyed, such as by subjecting the mold to heat or a solvent for the pattern material or both, to form the mold cavities.
In order to obtain the maximum yield of investment castings from a mold, it is desirable to attach as many patterns around the center tree or sprue as possible. The number of patterns of a given size that can be attached to a center tree having a given height depends upon the circumference of the center tree which is directly proportional to its diameter. The larger the diameter is of a center tree, the more patterns that can be clustered about it.
The size to which the diameter of a center tree can be increased to accommodate more castings is limited by the fact that the volume of metal required to fill the sprue passage in a mold increases as the square of the diameter of the center tree used to form the passage. In fact, it can be readily shown that the ratio (R) of the volume (V) of a mold sprue passage formed by a center tree to the area (A) of the center tree available for gating patterns increases directly with the tree diameter according to the relation R = V/A = D/4. This means that an increase in the center tree diameter can quickly lead to an excessive ratio of scrap sprue metal to usable castings which outweighs any advantage of being able to cluster more patterns around the center tree and makes the large diameter center tree impractical from an economic standpoint.
Another important consideration in making a pattern set-up is the spacing between the casting patterns around the center tree. The patterns must not be so close together as to prevent proper formation of the refractory investment mold or solidification of the metal in the mold cavities, etc. Because of a particular size or shape of the patterns, it has been possible in some situations to cluster only a few patterns around the center tree by conventional techniques. An increase in the tree diameter in order to accommodate more patterns while retaining the required spacing between them leads to an excessive R ratio for the reasons discussed above. An increase in the lengths of the pattern gates in an attempt to provide more room for patterns around the center tree is often unsatisfactory because the longer gates reduce the metal feeding ability of the sprue and can result in unsound castings.